[C++] Why virtual destructors

To explain the difference of virtual destructor and normal destructor, look at this two examples.

Normal Destructor

Code

class Enemy {
public:
    ~Enemy() { cout << "Enemy Destructor." << endl; }
};

class EnemyBoss : public Enemy {
public:
    ~EnemyBoss() { cout << "Enemy Boss Destructor." << endl; }
};

int main() {
    Enemy *e = new EnemyBoss();
    delete e; // Destroy the object
    return 0;
}

Output

Enemy Destructor.

Virtual destructor.

Code

class Enemy {
public:
    virtual ~Enemy() { cout << "Enemy Destructor." << endl; }
};

class EnemyBoss : public Enemy {
public:
    ~EnemyBoss() { cout << "Enemy Boss Destructor." << endl; }
};

int main() {
    Enemy *e = new EnemyBoss();
    delete e;
    return 0;
}

Output

Enemy Boss Destructor.
Enemy Destructor.

Conclusion

The virtual destructor is very useful when delete a instance of a derived class through the pointer of parent class.

When delete a instance of a derived class through the pointer of parent class, the virtual destructor will call the parent destructor, but the non-virtual destructor will not call the parent destructor. Therefore the second situation may potentially have the problem of memory leak.